The invention relates to a process for the formation of pores of controlled shape, dimensions and distribution in a polymer matrix. It also relates to a proton exchange membrane fuel cell.
The formation of pores of controlled shape, dimensions and distribution in a polymer matrix is important in a large number of applications, such as those employing electrochemical devices (sensor, battery, and the like) or any system requiring a porous material, such as filters.
In particular, it is important in the manufacture of the active layers of a proton exchange membrane fuel cell.
The active layers of proton exchange membrane fuel cells are the site of electrochemical reactions, that is to say of oxidation of hydrogen (for the anode) and of reduction of oxygen (for the cathode), which result in the production of water. These reactions take place in regions where a catalyst, which makes possible the acceleration of the reaction kinetics, an electron conductor, for collecting the electrons, a proton conductor and the gaseous reactants coexist.
The active layer is thus a composite material which has to be:                proton conducting, in order to make possible the transportation of the protons from the membrane to the reactive sites,        electron conducting, in order to make possible the transportation of the electrons from the reactive sites to the current collectors,        porous, in order to make possible:        a. access of the gases from the monopolar plates to the reactive sites,        b. evacuation of the water from the reactive sites to the monopolar plates.        
The active layers are the site of coupling of the phenomena of transportation of electrons, protons, gases and liquid water.
In an active layer, the electroactive surface area has to be as great as possible for a given geometric surface area and a given catalyst loading in order to obtain the most advantageous performance.
Currently, the catalyst employed is generally platinum and is provided in the form of spherical particles, the diameter of which is of the order of a few nanometers in order to increase the catalytic surface area as much as possible for a given weight of platinum. These catalyst particles are deposited on carbon particles, the diameter of which is of the order of a few tens of nanometers (from 20 to 80 nm inclusive), which can exist in the form of agglomerates. The catalyst can also be in the form of a plan or structured film. The combined product is generally known as “platinized carbon” or “Pt/C”. This conducting substrate was chosen due to its chemical stability and its cost. The proton conductor is an ionomer, that is to say a polymer electrolyte (for example of perfluorosulfonated type). Mixing these components results in a porous structure.
Usually, the active layers are prepared in two different ways:                the ionomer and the platinized carbon are suspended in solvents. This suspension, known as ink, is subsequently deposited on the membrane or on the diffusion layer in order to form the active layers after evaporation of the solvents. The structure obtained is porous.        the ionomer is impregnated (for example by spraying) on a premanufactured porous layer comprising the platinized carbon and a polymer binder which is not a proton conductor.        
From a structural viewpoint, in these active layers, the ionomer covers the particles of platinized carbon. Consequently, the gases have to pass through the ionomer before reaching the reactive sites. This has the consequence of limiting the access of the gases to the catalytic sites and thus of reducing the performance of the cell.
Furthermore, the distribution of the ionomer and of the platinum is poorly controlled and can result in poor use of the catalyst.
Finally, it is difficult to control the structure of the active layer (diameter of the pores, distribution, electroactive surface area) and thus of the electroactive surface with these manufacturing methods, which are imposed by the nature of the catalyst employed.